Illustrated herein in embodiments are liquid crystal displays and, more specifically, bistable cholesteric liquid crystal displays (LCDs).
Cholesteric liquid crystal displays have attracted attention in recent years as an electronic paper (e-paper) type of display device. The cholesteric liquid crystal display is capable of utilizing reflection from surrounding lights as a light source and has a storage property which can hold display contents after the supply voltage is turned off. Further, because an active matrix is not needed for driving the display, cholesteric liquid crystal display devices are capable of providing large-capacity displays, and may use a flexible substrate which is particularly suitable for e-paper.
The cholesteric liquid crystal is made up of spirally oriented rod-like molecules (mesogens), and exhibits a selective reflection phenomenon that reflects a light of a wavelength corresponding to a spiral pitch. The cholesteric liquid crystal display elements utilize this phenomenon. The cholesteric molecules can, under appropriate conditions, be in one of several different general orientations, namely, a planar orientation, a finger-print orientation, a focal-conic orientation, and a homeotropic orientation, as shown in FIGS. 1A–1B respectively.
The planar orientation shown in FIG. 1A is a state in which the cholesteric molecules 10 are aligned between two substrates 12, 14 in a helical spiral axis oriented vertical to the substrate plane. Cholesteric liquid crystals in the planar texture possess the optical property of separating incident white light into its left and right-hand circular components by reflecting one component and transmitting the other. This is due to the regular helical alignment of the cholesteric molecules in a spatially periodic twisted helical structure. For suitably chosen pitches, the reflected component is in the visible range, i.e., from about 400 nm to about 730 nm, giving rise to a selective color being observed.
The finger-print orientation shown in FIG. 1B is a state in which the spiral axis is oriented essentially parallel to the substrate plane. In practice, because of the anchoring effect of the surfaces of the cell, there are defects in this orientation and the helical axes of the domains may be more or less randomly oriented through the cell. This state is referred to as focal-conic and is schematically shown in FIG. 1C. The focal-conic state is made up of multiple domains, each having the same helical pitch, with the helical axes arranged approximately parallel to the substrates. In the focal-conic state, the cell is weakly scattering (nearly transparent) and transmits most of the incident light. When the bottom of the display is coated with a light absorptive layer 16, the color of the absorptive layer 16 is observed, which is usually black.
The homeotropic orientation shown in FIG. 1D is a state in which the spiral structure is decomposed and the cholesteric molecules 10 are oriented perpendicular to the substrate plane, also in a colorless state, and the color of the light absorptive layer 16 is observed.
To date cholesteric display technology has frequently employed switching between the planar state and the focal-conic state by means of electric fields being applied between electrodes 18, 20 affixed or adjacent to the substrates 12, 14. To obtain the reflective colored state, a relatively high voltage is necessary, up to 100–200 V for example, depending on the liquid crystal and the thickness of the display. For this reason, it is difficult to use such a display for fabrication of electronic paper, which ideally should use a low voltage for switching, preferably low enough that the display could be operated utilizing a battery power source for example.
Therefore, there is a need for a means of switching a cholesteric display with a lower voltage than presently required, thereby providing a more practical embodiment of electronic paper which consumes less power than present designs.